A Traffic Collision Avoidance System (TCAS) is an electronic system installed in aircraft that searches for and alerts pilots as to the presence of other aircrafts, including generation of actions to avoid approaching aircraft. TCAS operates independently of ground based equipment to give audible and visual warnings when there is a threat of collision with another aircraft. In conjunction with TCAS systems, transponders respond to interrogation with a special code that specifically identifies the aircraft on which it is located. Certain transponders have the ability to automatically transmit the altitude of the aircraft in addition to the special code. Hereinafter TCAS/Transponder based active surveillance systems are referred to as Traffic Surveillance Units—TCUs.
TCU-based active surveillance relative bearing estimates are accurate if the transponder is sending automatic dependent surveillance broadcast (ADS-B) data, measuring latitude and longitude. When ADS-B data is unavailable, TCU-based surveillance bearing estimates are limited in determining the bearing accurately due to many causes, both internal and external to the system. Accurate bearing measurements are important for navigational purposes. For example, bearing is required for autopilot station keeping, display, and taxing.
Therefore, there is a need to accurately measure bearing when ADS-B data is unavailable.